This invention relates to an emissions sampling system, and more particularly, the invention relates to an active filter temperature control method and apparatus for controlling the temperature of a particulate filter.
Emissions sampling systems are used to measure the products of combustion of a vehicle engine or other power plant to determine compliance with Federal regulations. Particulate matter is a product of combustion measured using a filter that collects the particulate matter during an emissions sampling test. Typically several filters elements are placed in series with a diluted exhaust gas sample stream to capture the particulate matter. The weight of the empty filter prior to the test is subtracted from the weight of the loaded filter subsequent to the test to determine the particulate matter emitted from the tested power plant.
The temperature at the filter sampling point has been found to strongly influence the accuracy of the test. Particulate test regulations are being adopted that require a tight temperature range at the filter, for example 47° C. Plus or minus 5° C. Some European standards may require the filter temperature to be maintained at 25° C. Plus or minus 5° C. The prior art has attempted to control the temperature in the area of the filter in several ways. For example, the filter temperature is raised to achieve a desired temperature by a heat exchange process in which a length of tubing is heated.
Utilizing a mechanical heat exchanged process for either the sample exhaust gas tubing or the diluted exhaust gas tubing requires a certain amount of surface area to achieve proper heat exchange. However, diffusion losses occur as particulate matter collects on the walls of the tubing. Any particulate matter collected on the walls will not be collected by the filter thereby adversely affecting the accuracy of the test. Accordingly, it is important to reduce the surface area as much as possible to prevent diffusion losses.
In another method, the temperature of the dilution gas is chilled to 15° C. Or some other fixed temperature to lower the temperature in the area of the filter. The fixed temperature of the dilution gas is determined based upon a maximum exhaust gas temperature when combined with the chilled dilution gas would achieve the desired temperature in the area of the filter. However, the temperature in the area of the filter is undesirably low and out of range for other exhaust gas temperature conditions.
Another prior art method to control the temperature in area of the filter is to adjust the portion of dilution gas mixed with the sample of the exhaust gas to change the temperature. However, regulations only permit the dilution ratio to be adjusted a very small amount during the test. Since the dilution ratio must remain somewhat fixed during the test, the dilution ratio must be optimized for one specific exhaust gas temperature, which is typically the maximum exhaust gas temperature. Again, the temperature in the area of the filter is too low for other exhaust gas temperatures during the test.
Therefore, what is needed is an active filter temperature control system and method enabling the temperature in the area of the filter to be maintained within a relatively tight range without adversely affecting the accuracy of the test.